Optical Polymers - American Chemical Society

Chapter 9 H a r d Plastic Claddings: Nearing T w o Decades of Performance

Downloaded by OHIO STATE UNIV LIBRARIES on September 1, 2012 | http://pubs.acs.org Publication Date: September 13, 2001 | doi: 10.1021/bk-2001-0795.ch009

Bolesh J. Skutnik

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Β J Associates, 51 Bunbury Lane, West Hartford, CT 06107 Current address: Fiber Optic Fabrications, Inc., 515 Shaker Road, East Longmeadow, M A 01028 Hard Plastic Clad Silica (HPCS) opticalfiberswere invented about two decades ago to improve on the original Plastic Clad Silica (PCS) optical fibers. General properties which materials need to have in order to be good cladding materials for optical fibers are discussed. Details of the invention/innovation process for HPCS are reviewed along with the development of this new type of opticalfiberstructure. A compilation of the several types now offered in the USA, Japan and Europe is presented. Material suppliers are identified. The advantages and limitations of these fibers for a wide range of medical applications are reviewed. Finally future developments and expanded products are suggested.

Introduction to Polymer Clad Silica Optical Fibers Before detailing the development and results of Hard Plastic Clad Silica fibers, we will review earlier polymeric cladding materials and in general discuss the requirements on a material to be useful as a cladding. The key optical properties a polymeric material needs to qualify as a fiber optic cladding are the following two: (a ) the refractive index, n of the polymeric material must be lower than that of silica core material at the operating wavelength and temperatures; and (b ) the optical transmission of the polymeric material should be high over the range of operating wavelengths and temperatures, i.e. low attenuation under operating conditions. Corollary properties to these are: (c ) the rate of change in refractive index with temperature must not be too extreme compared to that of the silica core material; is

© 2001 American Chemical Society

In Optical Polymers; Harmon, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.

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(d ) lack of significant dimensional transitions over the operating temperature range, even secondary or tertiary ones; (e ) regardless of whether it is physical, mechanical or chemical, good adhesion to the silica core material is desired; and (f ) ability to be applied and 'cured' in line with the drawing of the silica core material into optical fiber. Many polymeric materials were investigated during the 1970s and early 1980s as possible candidates for fiber optic claddings. Since the refractive index of the polymeric cladding must always be below that of the core material, silica here, there are very few materials that can satisfy this condition. Silicones and fluorinated polymers are the primary candidates, even though many low molecular weight organics have refractive indices below that of silica. The later being 1.4356 at the sodium D-line. With the exception of thermally cured silicones most of the other materials never routinely appeared in commercial optical fibers. Early researchers believed a soft buffer layer was needed between the optical fiber and a tougher outer jacket. In many ways the soft silicones, with their refractive indices, generally good transparency, and often very low glass transition temperatures, seemed a good answer. Early on the preference for methyl silicones over phenyl containing silicones was seen. The refractive index of phenyl silicones is higher than that of methyl ones and it rapidly increased to that of pure silica as the temperature approached 0°C or below. Secondly, throughout the normal operating range the transmission of methyl silicones was generally better than for the phenyl containing silicones. Plastic clad silica (PCS) opticalfibersrapidlycame to denote silicone clad silica fibers. The limitations of the PCS optical fibers soon became apparent as compared to silica/silica optical fibers. PCS optical fibers, especially those with hard outer jackets, increased in attenuation as operating temperature dropped, particularly below -20 °C. In fact the optical fibers became black, i.e. non-transmitting, at temperatures approaching -55 °C. The increase in attenuation with decreasing temperature was primarily due to a dropping numerical aperture of the core/clad couple which forced more core modes to become cladding modes that were more easily stripped by the outer jackets. A somewhat more insidious problem with the silicones is related to their softness. To properly terminate optical fibers, generally a connector must be attached to the fiber's end. For best performance the end usually must be polished to a high degree. These points lead to the need for a solid mating of the connector and the optical fiber so that chatterfreepolishing may be accomplished. Compression of the silicone in the termination is to be avoided because this would haphazardly increase the refractive index, possibly raising it over that of the silica core. Bonding to silicone is also very difficult because of its low surface energy. Standard termination procedure came to require the removal of thé silicone at the termination and the application of a epoxy or other adhesive to bond the optical fiber to the connector. As time went on it became apparent that often it was extremely difficult to get reliable, consistent bonding of the fiber and connector. The culprit was In Optical Polymers; Harmon, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.


131 determined to be residual low molecular weight silicone on the optical fiber after the majority of silicone was stripped off. Not only bonding was affected. In line connections appeared to degrade over time, increased attenuation, especially when high power light transmission and/or low pressure conditions existed. The low molecular weight silicone would be drawn into the connection space between the fiber ends; deposited on the ends; and then degraded as high power light became partially absorbed in the coating. Silicones won out over thefluorinatedmaterials of the time, because they could be applied inline as the fiber was drawn, they could function as buffers, and they did adhere modestly well to the silica surface. Mostfluorinatedmaterials were polymers that needed to be extruded onto the formingfiberswhich meant handling the silica fibers before any protective coatings were applied. This guaranteed a weak fiber with significant surface flaws. Secondly, the adhesion of most of the fluorinated polymers to a silica surface was quite poor. Lastly, because of many applications, which called for high to ultra high purity silicones, the latter were cleaner and thus generally had lower attenuations, higher light transmission, than the fluorinated materials. It was in this situation that the following innovations and inventions were conceived and developed.

Invention/Innovation of Hard Clad Silica Optical Fibers The concept of a hard plastic clad silica optical fiber arose in 1979. Within the Corporate Research Division of Ensign-Bickford Industries, Inc. [EBI], development work on the standard plastic clad silica [PCS] optical fibers - with soft silicone plastic as the cladding - had progressed to the point where the low temperature optical limitations and the problems with connectorization of the PCS fibers were seen to limit the use of these optical fibers in many applications. Silicone was used because its refractive index, as required for claddings, was below that of the pure silica used in the core of the optical fibers. Soft silicones seemed to be ideal to serve as a buffer coat as well as cladding for silica cores, protecting the latter from mechanical damage. While several silicones have very low glass transition temperatures, where they become hard, even the best of these materials have a minor secondary transition in the -40 to -50°C range that unfortunately greatly increases their refractive index within this temperature range. The attenuation of the fibers thus rises very fast through this temperature range and the fiber becomes unusable near -50°C. To achieve good, thermally stable terminations, problems arise from the need to remove the soft cladding from the fiber and the need to remove all the residual material from the silica surface without compromising the surface. Following conception, I set as the initial design goals for the new optical fibers that they should: (a) have as low an attenuation as possible; (b) have a large numerical aperture, NA, which could be varied; (c) be insensitive to temperatures, particularly to -50°C and below; (d) be as radiation hard as PCS fibers; (e) be easier to reliably terminate than PCS fibers; and (f) be capable of being drawn faster than PCS fibers at that time. Each of these goals could be correlated with one or more cladding material properties. Table I presents the results, obtained with Hard Clad Silica [HCS®] opticalfibers,for each of the design/cladding property goals. In Optical Polymers; Harmon, J., et al.; ACS Symposium Series; American Chemical Society: Washington, DC, 2001.


132 Table I. Cladding Property Design Goals/Results HCS Fiber Property Desired Product Property Cladding Material Property 5-12 dB/km at 820nm Very Low Absorption 600Low Loss lOOOnm, Tightly Cured Comp. N A = 0.38 or 0.45 Low Refractive Index Large N A 1 dB/km Added Loss Hard, Glassy, Tg> Room Τ at-50°C or Very Broad Tg Temperature Insensitive Induced Losses Below Not Interfere with Pure 10 dB/km at lOkrads Silica Core Property Radiation Hard Crimp/Cleave Possible Cure to Hard, Adherent Better Connectorization Glassy Material 2-3 Times Faster than UV-Curable Composition Typical Silicone PCS Faster Processing Preliminary experiments began in 1980 and by 1981-82 the first HCS fibers were produced and sold. A market study was commissioned and the results indicated a potentially expanding market compared to the standard PCS fiber. The new HCS fibers were described in papers ' presented at the Society of Plastic Engineers ANTEC '83 and at the Optical Society of America CLEO '83 meetings. Besides the properties described in Table I, HCS fibers had an additional structural feature that for all core sizes the cladding thickness was only 7 to 20 μ. This feature as well as a few others, which we will describe later, is important for many medical applications. Returning to the innovation story, Ensign-Bickford Optics Company was formed in 1984 to produce and market HCS optical fibers. The basic patent for HCS fibers issued in early 1985 and was followed by a second patent in 1987 that described an improved HCS fiber and an HCS-type coated all-silica optical fiber. Having established a beachhead, other workers in the United States and in Japan began work on their own versions of HPCS optical fibers, which culminated in 3 M Specialty Optical Fibers obtaining a patent for their TECS® optical fiber and in Dainippon Ink & Chemical Company obtaining a patent for their improvement on the basic HPCS cladding, both in 1989. The latter has given rise to a variety of HPCS type fibers in Japan and Europe. The information on United States patents is given in Table Π. 1 2

Number 4,511,209 4,707,076 4,852,969 4,884,866 5,203,896 5,302,316 5,690,863

Table II. US Patents for HPCS Materials Date Issued Fiber/Producer April 16,1985 HCS® /Ensign-Bickford Optics [EBO] Nov. 17, 1987 HCS® +HCS® All-Silica Fibers/ EBO Aug. 1, 1989 TECS® / 3M Specialty Optical Fibers Dec. 5, 1989 HPCS generic fibers/ DIC material April 20, 1993 Optran®/CeramOptec Industries April 12, 1994 HPCS generic fibers/DIC material Nov. 25, 1997 ?/Optical Polym. Res. material 3

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133 The formulations for the various HPCS materials are proprietary, but as these patents describe the common thread is to use highly fluorinated esters of acrylic or methacrylic acid with crosslinking multi-functional acrylates and methacrylates, generally photinitiators and commonly adhesion promoters. In some cases catalysts or thermal curing additives have been included in the cladding precursor materials Monomers and oligimers of thefluoroacrylatesare used as the base polymer forming component. The adhesion promoters, where present, generally are materials which bond easily to silica surfaces and are compatible or attracted to the basic polymeric backbone.

Properties of HCS® Optical Fibers HCS optical fiber is of the step-index type. A pure silica rod is drawn to the desired core diameter, then the proprietary hard polymer optical cladding is immediately applied to the pristine surface of the silica core. The polymer is cured with uv radiation and bonded to the silica surface. The cladding is very hard [Shore D 70] and since it is bonded well to the silica surface, it need not be removed when terminated. The cladding is typically about 10μ in thickness which leads to very high core/clad ratios, from 80% to 94%. These three properties - hardness, adhesion to core, high core/clad ratio - are the unique structural properties of HCS and the whole class of HPCS fibers. A number of mechanical and optical properties differentiated the original HCS fibers from other fibers available at that time. Among the most significant are high tensile strength and high resistance to static fatigue . Dynamic tensile strength for 10m gage lengths is typically above 750 kpsi (>5 GPa) with very high Weibull slopes, m> 60, i.e. extremely sharp Weibull strength plots. This provides safety margins in critical applications such as medical ones. The static fatigue parameter, n, which is related to the slope of log/log plot of time to failure versus failure stress, is quite high even for fibers immersed in water and lies typically in the 25-30 range. This allows the fiber to safely sustain tight bend radii in applications, as for example in entry to blood vessels through cannula. The high core/clad ratio of HCS fibers is typically 87% to 94% for the fiber sizes employed in biosensing, endoscopy or laser surgery. Its main advantages are to allow a higher energy density for a given core diameter, provide increased coupling efficiency and to allow easier connectorization. The low attenuation over a reasonably broad wavelength region, down to 4 dB/km @ 800nm, makes the fibers useful for sensitive biosensing applications and for a number of medical laser sources. HCS fibers can have numerical apertures [NA] of about 0.4 or 0.5 due to the low refractive index of the hard, highlyfluorinatedcharacter of the cladding. The high NA provides two main benefits, increased coupling efficiency and low bending loss sensitivity. The high coupling efficiency is especially useful for biosensing and illumination applications and the good bending loss sensitivity can be important for all medical, military and industrial applications. The NA does decrease over length 10



134 as it does for all polymeric clad optical fibers to an equilibrium value, but for the typical 3-5 m lengths used in medical applications the NA value remains constant and high. The hard, adherent polymeric cladding provides several benefits to HCS fibers. Pistoning in the connector is prevented and the cladding protects unbuffered sections from damage. Since the cladding isn't removed during termination, the reliability of connectors is enhanced. Finally because of these properties it is possible to make reliable crimp & cleave connections, which have the potential of reduced cost and automated assembly. The HCS optical fibers function well over a wide range of temperatures. Samples of fiber exposed to liquid nitrogen temperatures, -196C [77K], and below were used to cany spectroscopic information from materials held at these temperatures . In the other direction, HCS fibers are stated to be usable up to 125°C, essentially continuously. Of more interest for medical applications is the fact that the fatigue behavior of thesefibers,even in highly moist environments, remains predictable and unchanged from ambient water to steam exposure at a temperature of 121°C. The strength at the higher temperatures is reduced somewhat but in a predictable fashion, and it remains high as compared to other fiber types. The radiation behavior of the standard HCS fibers and special radiation grade fibers are essentially related to the silica materials used in the core . Short and longer term transient behavior have been reported at 865nm and at 1300nm under a number of environmental conditions. These tests were primarily done for military applications, where the dose levels are much lower than for sterilization. All pure silica core fibers with good behavior in this region typically gain little or no additional attenuation as the dose level goes to the 2.5 megarad sterilization dose, assuming that the fiber is illuminated for a period before its final use. 11

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Producers/Properties of Other HPCS Optical Fibers As mentioned earlier, within the first few years several more producers of HPCS type fibers have come on the scene. Each has their own variation of the cladding as indicated by the patents listed above in Table II. The main players from around the world are HCS® by Spectran Specialty Optics and Toray Fiber Optics; Optran® by CeramOptec Industries and TECS® by 3M Specialty Optics. In Japan the major emphasis has been fiber for the data link and power supply applications, rather than any significant effort in medical applications. Recently, Fiber Optic Fabrications has begun evaluating the materials available through Optical Polymer Research (OPR). These new materials are available in a variety of viscosities and refractive indices. Preliminary results on clad only optical fibers point to some equivalency with other HPCS clad only optical fibers. Initially the optical properties of the OPR clad fibers, while similar to other HPCS fibers, are not equivalent. Testing and evaluation continues.


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Advantages/Limitations for Medical Applications A number of the properties of HPCS fibers make them very desirable for medical applications, both for laser surgery and for in-Vivo sensing/endoscopy. These are summarized in Table III for the common advantages. High core/clad ratios, low loss, smaMarge core sizes and broad transmission window are of value for different reasons in each of the application areas. Numerous papers, primarily by EBOC and by 3M at SPIE meetings, have presented and discussed how the different properties are important in various medical applications " .


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Table m Advantages ofHPCS Fibers for All Medical Applications Tighter Bends Permitted in Use High Inherent Strength and and Packaging & Storage Easier Good Fatigue Behavior Apply Connectors Directly Over Clad Hard, Thin, Adherent Fiber; Improved Reliability Cladding Potentially Less Expensive, Crimp & Cleave Connectors High Volume, Disposable Devices on Proximal End Diminished Danger in Long Operations Cladding Non-Thrombogenic Easier to Sterilize & to Keep Sterile Low Bioload Materials As this table shows there are many good reasons for choosing HPCS optical fibers for use in medical devices. However, as with all things, there are also limitations to the currently available products. These are summarized below. In the next section we shall mention developing products, which are addressing some of these limitations. In laser surgery applications there are two primary limitations. At very high laser powers, the cladding will vaporize, particularly near the fiber ends. This limitation is due to the thermal stability of the proprietary cladding material, to the planarity of the end face, and to minor misalignment of the laser/fiber interface. The ultimate limiting factor is the first one, because to improvements in the mechanical problems will eventually be limited to the material thermal properties. The use of lasers operating at wavelengths below 400 nm, the near uv region, is hampered by the fact that the hard cladding generally absorbs light strongly in this region of the spectrum. Since most hard claddings are uv cured, they contain uv absorbers, i.e. the photo initiators which start the cure process reactions. Other components of the hard clad compositions may also have major uv absorption characteristics. The absorption in this region increases much more rapidly for the hard cladding than for silica, as is true for most plastic materials. In the biosensing/endoscopic applications, there are three areas where limitations become evident. A large N A is advantageous for these applications, however for the higher NA HPCS fibers, the hardness, wear resistance and overall robustness typically is reduced from that of the "standard" N A product, making



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thesefibersmore susceptible to damagefromhandling. This is thought due to the need to employ very highlyfluorinatedcomponents, and the limited availability of such chemicals, particularly as polyfunctional aciylates. Another limitation to increased sensitivity is that for core dimensions smaller than 200 μ the attenuation begins to increase rapidly across the spectrum, because more power travels in the cladding as the core size approaches or drops below 100 μ. The hard plastic clad is more lossy than silica and physics indicates that the signal can extend beyond even the cladding when it is thin and the core size drops below 100 μ. Finally some biosensor systems cannot adhere well to the cladding. This may require the removal of the cladding which is hampered if the cladding adheres too well to the silica core.

Summary Hard plastic clad silica opticalfiberswere invented nearly two decades ago in an attempt to achieve the benefits of both all silica and silica/siliconefiberconstructions while minimizing each type's problems. To a great extent the original HCS® fibers did indeed meet these intentions. Thefirstaim in 1981 was to provide plastic clad pure silica core fibers with improved thermal and optical behavior for military markets. The thin, hard clad structure was proven to yield high strengthfiberswith reliable and predictable fatigue properties. This lead to expanding the market to datacom and particularly to medical applications. The mechanical, optical and structural properties have been found to be especially useful in the design of laser fiber delivery systems and in the design of endoscopic and biosensing systems. From one nascent supplier with only two variations, HPCSfibersare now produced in varying 'flavors' by suppliers in the United States, Europe and Japan. New, improved varieties of HPCS fibers are being developed by different fiber manufacturers to expand the capabilities of this interesting class of optical fibers. The invention of the hard cladding with the possibility of formulating changes in many of the optical and thermal properties and thus producing new fibers has vitalized the general plastic clad silicafibermarket.

References 1. B.J. Skutnik, ANTEC '83 Proc.1983, 436 (Society of Plastics Engineers). 2. B.J. Skutnik and R.E. Hille, CLEO '83 Proa, 1983, (Optical Society of America). 3. B.J. Skutnik, US Patent # 4,511,209 (1985). 4. B.J. Skutnik and H.L. Brielmann, Jr., US Patent # 4,707,076 (1987). 5. S.A. Babirad, F. Bacon, S.M. Heilmann, L.R. Krepski, A.S. Kuzma, and J.K. Rasmussen, US Patent # 4,852,969 (1989). 6. Y. Hashimoto, M. Kamei and T. Umaba, US Patent # 4,884,866 (1989). 7. W. Neuberger, US Patent # 5,203,896 (1993). 8. Y. Hashimoto, J. Shirakami and M. Kamei, US Patent # 5,302,316 (1994). 9. P.D. Schuman, US Patent # 5,690,863 (1997).



137 10. W.B. Beck, M.H. Hodge, B.J. Skutnik and D.K. Nath, EFOC/LAN Proc., 1985 145 (Information Gatekeepers Inc.). 11. Schwab, S.D. and McCreery, R.L., Anal. Chem. 1984, 56, 2199. 12. Skutnik, B.J., Hodge, M.H. and Clarkin, J.P., SPIE 1988, 842, 162. 13. Skutnik, B.J. and HiIle,R.E., SPIE 1984, 506, 184. 14. Skutnik, B.J., Greenwell, R.A. and Scott, D.M., SPIE 1988, 992, 24. 15. Evans, B.D. and Skutnik, B.J., SPIE 1989, 1174, 68. 16. McCann, B.P., SPIE 1991, 1420, 116. 17. Skutnik, B.J., Hodge, M.H. and Beck, W.B., SPIE 1987, 787, 8. 18. Skutnik, B.J., Hodge, M.H. and Clarkin,J.P.,SPIE 1988, 906, 21. 19. Skutnik, B.J., Brucker, C.T. and Clarkin, J.P., ibid. 244. 20. Skutnik, B.J., Clarkin, J.P. and Hille, R.E., SPIE 1989, 1067, 22. 21. Skutnik, B.J., Hodge, M.H. and Clarkin, J.P., ibid., 211. 22. Skutnik, B.J., SPIE 1990, 1201, 222. 23. McCann, B.P., Photonics Spectra 1990, 24 (5), 127. 24. Krohn, D.A., Maklad, M.S. and Bacon, F., SPIE 1991, 1420, 126. 25. McCann, B.P. and Magera, J.J., SPIE 1992, 1649, 2. 26.Skutnik, B., Neuberger, W., Castro, J., Lashinin, V.P., Blinov, L.M., Konov, V.I. and Artjushenko, V.G., ibid., 55.


Optical Polymers - American Chemical Society

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